1. Field of the Invention
The present invention relates generally to laser technology for photolithography, and, more particularly, to optimization of extreme ultraviolet (EUV) light production.
2. Description of the Prior Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site.
The line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may be mixed or dispersed with another material such as a liquid. Delivering this target material and the laser beam simultaneously to a desired irradiation site (e.g., a primary focal spot) within an LPP EUV source plasma chamber for plasma initiation presents certain timing and control problems. Specifically, it is necessary for the laser beam to be focused on a position through which the target material will pass and timed so as to intersect the target material when it passes through that position in order to hit the target properly to obtain a good plasma, and thus, good EUV light.
A droplet generator heats the target material and extrudes the heated target material as droplets which travel along an x-axis of the primary focal spot to intersect the laser beam traveling along a z-axis of the primary focal spot. Ideally, the droplets are targeted to pass through the primary focal spot. When the laser beam hits the droplets at the primary focal spot, EUV light output is maximized.
When the laser fires, however, plasma formed from preceding droplets within a burst interferes with trajectories of succeeding droplets within the burst, pushing the droplets out of the x-axis of the primary focal spot. The result is that the droplets are displaced (“pushed-out”) along the y- and/or z-axes away from the primary focal spot when hit by the laser beam. This push-out ramps up rapidly (e.g., in about 15-20 ms) and can be quite large (e.g., 120 μm displacement from the primary focal spot). The large and rapid nature of the push-out is especially problematic during continuous mode firing of the EUV system because re-alignment of droplets to the primary focal spot cannot be achieved before the laser fires again and lases a succeeding droplet outside the primary focal spot. Thus, the effect of the push-out is that plasma generated from succeeding droplets is not focused in the primary focal spot of the collector, and, consequently, EUV light output is not optimized.
Current methods to compensate for droplet push-out rely on droplet-to-droplet feedback control of the droplet generator to re-align droplets in the primary focal spot after the push-out has occurred. Such droplet-to-droplet feedback control is not ideal, however, because of the relatively long time necessary to re-align droplets relative to the speed at which the droplets travel. For example, when the laser is firing in a continuous mode, the droplet-to-droplet feedback after plasma from a first droplet causes a push-out disturbance is too slow to completely re-align a next droplet to the primary focal spot target before that next droplet is hit by the laser beam.
What is needed therefore is an improved way to accurately re-position the droplets of target material more rapidly so the laser beam strikes the droplets within the focal spot of the laser beam.